The acute problem of enforcing abstinence in cases of severe drug abuse may be resolved through administration of compounds which eliminate the reinforcing effects of the abused drugs. Compounds serving as antagonists to behaviorally active drugs must permit normal functioning with respect to non-drug related contingencies and reinforcers. Optimally, the antagonists compounds should eliminate the reinforcing effects of a range of drugs which have high abuse liability. In the present investigation monkeys will serve as subjects for evaluation of effects of antagonists to commonly abused behaviorally active drugs. Oral self-administration will be established using a capsule dispensing system which permits study of drug effects in preparation form and route of administration analogous to the most common pattern of drug abuse in humans. Several compounds will be evaluated for effectiveness as antagonists to the reinforcing effects of amphetamines and barbiturates. Evaluation will include determination of effects of antagonists on self-administration of single drugs and combinations of drugs. Consideration will also be given to whether the antagonists eliminate self-administration of classes of drugs or whether the effect is one of alteration of drug preference. Additionally the behaviorally active drugs, antagonists and combinations of the two, will be evaluated for adverse effects on behavior maintained by other reinforcers.